1. Field of the Invention
This invention relates generally to humidity sensors and, more particularly, to such sensors whose electrical resistance varies in response to changes in humidity in the surrounding atmosphere.
2. Brief Description of the Prior Art
Humidity sensors are generally electrical components which display a parameter change in response to change in humidity of the surrounding atmosphere. A known problem with humidity sensors is long-term drift during exposure to various environmental conditions including certain chemicals.
It is well known that humidity sensors can be fabricated using polyelectrolyte films as humidity dependent A-C resistors. Examples of such prior art are provided in Pat. No. 4,386,336 and Japanese Patent No. 59-44647.
A polyelectrolyte material which is very attractive for humidity sensing because of its excellent thermal and chemical stability is a sulfonated fluoropolymer manufactured by DuPont under the trademark Nafion. Nafion 117 is a polytetrafluoroethylene material with perfluorosulfonic acid groups appended thereto. Efforts to utilize this material for humidity sensing have been made in the past. Lawson (Pat. No. 4,083,765) describes an electrolytic hygrometer that utilizes a hollow fiber formed of Nafion having closely spaced metal electrodes on the inner and outer surfaces. This device is complex, expensive and requires flowing gas of controlled velocity. Laue (NASA Technical Briefs, 1985) used a solid film element with contacts on opposing faces as a humidity dependent capacitor in an RC oscillator circuit. Again, response is likely to be very slow because of the thickness of the film. Lawson et al. (Proceedings of the Sensors Exposition, Detroit, Sep. 15, 1987) described a polymer electrolyte hygrometer that utilized the relatively thick Nafion film embedded in silver epoxy for electrical contacts. Response time was faster because the sensing mechanism involved detecting changes in surface conduction due to adsorption of water on the outer surface of the strip. The device appeared to work well in a harsh environment, in particular, a paper mill, however it required complex and expensive electronics. Due to reliance upon surface conduction, the device may be susceptible to contamination, although this did not appear to be a serious problem in the paper mill.
Huang describes in Pat. No. 4,681,855 humidity sensors that utilize fluoropolymers similar to Nafion but with weak (e.g. carboxylic) as well as strong (sulfonic) acid groups appended to the fluoropolymer backbone. The advantage of using combined weak/strong acids was to have the ability to tailor the response characteristics for different applications (e.g., dry or moist atmosphere). Huang also contended that the purely sulfonated fluoropolymer did not possess sufficient sensitivity above 40% relative humidity and that adding the --COOH moiety improved the response at high humidity.